The present invention relates to a new and improved construction of an auxiliary apparatus for a particle analyser for measuring the particle count within particle size intervals which are bounded by displaceable or shiftable thresholds, and particularly relates to a particle analyser for the analysis of blood particles, wherein the particle analyser is equipped with feeler means or feelers which generate an electrical signal corresponding to the size of the sensed particles.
It is extremely problematic to analyse a mixture of particles of different size depending upon the distribution of the prevailing classes. What is meant herein by the expression distribution as used in this disclosure, is a discrete distribution of body volume and, in the narrower sense, a histogram of the frequency of the body volumes. In the statistical sense the histogram or the distribution is the probability density that a feature or characteristic, as to its value, will lie within a predetermined interval. The characteristics or features can be of different nature, for instance physical, chemical, morphological and others. If interest is only expressed in the particle count of a certain class, which ideally is present isolated from the distribution of other classes, then the analysis is unambiguous. Prevailing errors usually are attributable to the employed equipment and the accuracy is only limited by the signal-to-noise ratio inherent to the employed system. However, as soon as there are present mixed or hybrid distributions, wherein, for instance, the same size particles of different particle species or classes belong to a respective inherent distribution density, then there are required criteria for discriminating between the overlapping distributions.
With the heretofore known particle analysis equipment there are employed the human efforts of the operator in order to separate certain ranges of a mixed distribution of the body volume. The operator evaluates the distribution spectrum, which is rendered visible in an appropriate and therefore here not further described manner, or a function derived therefrom, and based upon criteria which the operator selects determines a "separation threshold," below which, for instance, the particles are allocated in accordance with their size to one class and above which the particles are then allocated to the other class. The once selected threshold is impressed upon the system, the particle analyser then only detects, for instance, the signals related to the particles which are below or above this threshold. Assuming the signals below the threshold are predicated upon spurious particles, the signals above the threshold upon particles which must be analysed, then the set threshold constitutes a discriminator for spurious and useful signals. If there are measured distributions of a number of size classes, then there must be selected a correspondingly greater number of separation thresholds which must be impressed upon the measuring system, provided that the individual classes are satisfactoriy separated in order to be even able to detect a mixed distribution. This also is true for a bimodal mixed distribution.
Now if a particle analyser is employed for a special purpose, in other words for a limited field of application, say, for blood particle analysis, then for certain equipment designs the thresholds are fixed and cannot be set by external manipulations, in order to separate the particles which should be counted or measured from the particles which should not be incorporated into the measurement.
The setting of separation thresholds in a multimodal distribution curve, in the first instance leads to truncated distributions. The degree of truncation has a direct influence upon the integral over the distribution density, for instance upon the result of a count, and determines, usually dominantly, its accuracy. This is only valid if one stays with the truncated distribution without adequately correcting the same. If the distribution curve changes at the region of a fixed threshold, then by virtue of the increasing or decreasing truncation of the distribution to be analysed there is also altered the result of the analysis. While with prior faulty placement of the separation threshold it is possible for the result to become more accurate, normally however the opposite is true; the obtained result becomes poorer because the threshold previously usually was optimumly set. If, for instance, a particle analyser is designed for volume distribution analysis and for counting erythrocytes in human blood, that is to say, all of the sampled signals emanating from particles of a predetermined particle size interval should contribute to the measurement and a separation threshold should eliminate from the measurement those signals predicated upon artifacts, in other words, particles which are not erythrocytes, then this analyser, apart from possible exceptions, cannot be used, without correction of the threshold, for the counting of erythrocytes in animal blood. If the signal-to-noise ratio of the particle analyser is insufficient, then already the physiologically possible variation range of the cell sizes in human blood requires an individual accommodation of the threshold to each individual blood sample. Such analyser cannot be used at all for the analysis of just any random particles.
There will be clearly recognized from the foregoing the extremely narrow range of application with respect to a distribution function and, additionally, with insufficient signal-to-noise ratio the critical behavior of a particle analyser with separation thresholds which are fixedly set within the circuit design.
It is possible to construct particle analysers in such a manner that the operator, as required, can set the separation threshold or thresholds with the aid of a device mounted externally of the equipment. What previously was the task of an operator who was specially trained, now must be accomplished in equally exact and good quality by the particular random user of the equipment. The so-called setting or adjustment instructions should enable positive "setting" of a desired separation threshold by the user, without such manipulations falsifying the analysis results. Such setting instructions frequently are very simple, but, on the other hand, performance thereof is difficult and unreliable.
Thus, the threshold positioning or setting with the aid of an oscilloscope, where the particle signals in relation to the base noise of the artifacts are visible and can be approximately separated by varying a discriminator or threshold, delivers poorly reproducible values. Another recommended procedure requires the determination of a summation distribution curve. This is obtained by plotting counting results as a function of the threshold position. In the ideal case there is formed a horizontal segment, the so-called plateau, on the basis of which there can be set the threshold. The less the segment or plateau deviates from the horizontal and the greater its range, that much greater is the signal-to-noise ratio of the analyser. In the practical fields of application of blood particle analysis the plateau however, does not have any horizontal section and is also narrowly limited in range. Positioning of the threshold on the basis of the determined curve is unreliable. Furthermore, there must be considered the quite appreciable expenditure in time for the determination of the summation distribution curve, considering the fact that it must be plotted periodically and separately for erythrocytes and leucocytes. Additionally, the cell suspensions which are to be analysed are frequently unstable, something not known to many users. Consequently, the summation distribution curve is falsified and the threshold positioning based thereon is questionable. A possible solution from this dilemma is to improve the signal-to-noise ratio of the analysis system; with a higher investment in sensors and electronic hardware it is possible to obtain a sub-critical threshold positioning. A further possibility is to carry out the threshold setting according to the present invention.
In U.S. Pat. No. 3,638,227, granted Jan. 25, 1972, there is disclosed a plotter apparatus which automatically plots the particle count rate as a function of the threshold voltage. Based upon the obtained curves there are set the thresholds in accordance with qualitative criteria, leading to values which are poorly reproducible and dependent upon the dexterity of the operator.
In U.S. Pat. No. 3,557,352, granted Jan. 19, 1971, there is disclosed an auxiliary apparatus for a particle counting system, which determines, by conversion of signals of such system, a particle size which divides the examined system into two parts or fractions which have a certain relationship to one another. For instance, there can be found a mean value of a mass distribution. Yet, the invention disclosed therein does not provide any teaching for finding the minimum between two neighboring classes and such also would not be obvious to one skilled in the art.
In French Pat. No. 2,097,763 there is disclosed a system for receiving optical signals and for the evaluation thereof by a computer system. In particular, within an existing histogram there are detected those signals belonging to a certain class. The field of application of such invention is related to the automatic navigation of space ships. Utilization of this complex system, which has been particularly developed for the processing of optical signals, at an auxiliary device, which would be economical and capable of being fabricated in series, equally would not be obvious for one skilled in the art.